The invention relates to a method of compensating for eddy currents induced by the switching on and off magnetic fields in gradient coils in NMR imaging apparatuses, which method includes at least one compensation cycle comprising the following steps:                a) Performing at least one detection of the magnetic field generated by the current flowing in a gradient coil, i.e., a gradient field        b) Extrapolating the course of the effectively generated gradient field from the data of said at least one detection;        c) Generating a gradient fields excitation current compensating the eddy current effects on the gradient fields on the basis of the comparation of the effectively generated gradient fields and of the target gradient field.        
Gradient coils are coils that are used, for instance in NMR imaging apparatuses, for generating magnetic fields varying in space along a certain direction.
In order to execute as short imaging sequences as possible, the switching of and off of gradient coils is very fast. This induces fast changing exciting currents and generates eddy currents in the ascending and descending slopes of the switching currents, which hinder the generation of the desired magnetic field in the desired time. This has negative effects on the imaging quality and requires correction.
Eddy current induced magnetic fields are known to be time and space varying fields, that may be defined by a three-dimensional coordinate system, i.e., having three coordinates. The mathematical description of the field referred to the coordinates may consist in a polynomial expansion in which, for each of the three coordinates, zero orders, linear orders and nonlinear orders are provided.
Even when only zero and linear components are considered (zero components and one, i.e., linear, components, are of the zero and one order with respect to the coordinate) with respect to each coordinate, the presence of both orders prevents eddy current effects on the gradient coil field from being effectively compensated for, by only measuring these effects in a predetermined location in space or in a limited region of the whole volume being considered.
A single measurement, intended as a measurement of the field at a single point in the space permeated by the field would not allow to determine both zero and linear components, since any correction or compensation that only accounts for the information retrieved from a single measurement (as defined above) would lead to inaccurate results, in that the correction would only apply, on an average, to the whole partial space volume relative to that measurement and not in a wider volume, like the whole volume being considered.
Generally only one correction step comprising extrapolating the magnetic field of the gradient coils along each one of the three coordinates of spatial direction during excitation, comparing the extrapolated gradient fields with the theoretical ones and using the differences between extrapolated fields and theoretical fields for modifying the time dependent excitations currents of the gradient coils in such a way as to cancel or minimize the said differences, is not sufficient in order to fully compensate the effects on the gradient fields due to the eddy currents and the said steps has to be repeated a certain number of times.
Each repetition requests again the measurement of the gradient fields generated by the gradient coils. Since this measurement requests to carry out at least two measures at different points in the space permeated by the gradient field and since this two measures has to be repeated for each of the three coordinates defining the space, the compensation of the eddy currents requests very long time to be carried out since a certain satisfying level of compensation has been reached.
Furthermore if one considers that as taught by “Anlytical Method for the Compensation of Eddy-Current Effects Induced by Pulsed Magnetic Field Gradients in NMR Systema” by P.Jehenson, M. Westphal, N. Schuff, Journal of Magnetic Resonance, Vol 90, 1990 pages 264-278, the linear components of the magnetic fields corresponding to the gradient fields and the zero order components of the magnetic fields corresponding to a constant magnetic field has to be compensated separately by such a recursive method it clearly appears how important it is to reduce the time needed for carrying out the compensation proceeding.
The above mentioned document teaches to carry out compensation by separating the linear components of the field from the zero order components of the field and by applying a recursive compensation algorithm at each compensation cycle being necessary to carry out a new measurement of the effective gradient fields generated.